The present invention relates to a method and apparatus for providing power to an electric motor. More specifically, the present invention relates to a multi-stage power system providing a regulated DC voltage using low voltage batteries that may be conditioned by an inverter to drive an electric traction motor in a vehicle.
In today""s automotive market, there exist a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. The propulsion systems each have specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles. Hybrid vehicles are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. Hybrid vehicles, as compared to conventional vehicles driven by an ICE, offer improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor could shut down the ICE during a stopped or idle condition, allowing the electric motor initially to propel the vehicle and eventually restart the ICE, improving fuel economy and reducing emissions.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source into mechanical energy, turning a generator which converts the mechanical energy into electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and reduce emissions, as compared with a conventional ICE vehicle.
Secondary/rechargeable batteries are an important component of a hybrid vehicle system. Secondary batteries provide for the storage of energy which can be delivered to the wheels of a vehicle on demand. In addition, secondary batteries enable an electric motor/generator (MoGen) to store energy recovered during braking. Accordingly, the batteries provide a means of load balancing, absorbing or delivering the instantaneous difference in energy generated by the ICE with that required by driving conditions.
A battery module may be comprised of several series-connected electrochemical cells. Typical electrochemical cell voltages are in the one to two volt range. Present battery module output voltages are in the 12 to 42 volt range. Conventional vehicle traction systems operate with a DC bus voltage in the high range of substantially 300 to 400 volts. In conventional electric or hybrid vehicle applications, battery modules are stacked in series to provide the desired high DC voltage levels required by the high voltage vehicle traction system. Generally speaking, a high voltage vehicle traction system provides cost, performance and weight advantages, as compared to low voltage traction systems.
Series-connected battery packs complicate a vehicle traction system and affect the reliability of the traction system. The main difficulty with series-connected battery modules is in providing charge balancing to the individual cells comprising the battery modules. Charging and discharging a large number of series-connected cells with a current common to all cells results in poor charge balancing and accelerated aging, caused primarily by operating temperature differences between cells.
The present invention includes a method and apparatus to utilize a high voltage inverter motor set with low voltage battery modules. The present invention utilizes several power stages to provide a high voltage (substantially 300 to 400 volts) to the vehicle traction system. Each power stage includes a low voltage battery module and a bi-directional boost/buck DCxe2x80x94DC converter. The high voltage sides of the power stages are wired in parallel and connected to at least one voltage inverter and motor set such that the total power load is actively shared by the individual power stages. Each power stage has individual current control, with one overall voltage regulation loop controlling output voltage. The low voltage battery modules may be diode-ored to support miscellaneous low-voltage accessory power loads. In alternate embodiments of the present invention, the battery modules may be replaced with fuel cell power modules.
The present invention, in the preferred embodiment, further includes a vehicle having a parallel hybrid drive system incorporating a hybrid system controller executing the methods of the present invention and an internal combustion engine (ICE), but any vehicle utilizing an electric traction motor or MoGen is considered within the scope of the present invention. The MoGen of the present invention not only provides for propulsion of the vehicle during certain vehicle operating conditions, but also replaces an alternator to charge the battery pack in the vehicle and thus replaces a conventional starter motor to start the ICE. The hybrid system controller of the present invention will utilize the ICE and MoGen to propel or motor the vehicle in a manner that will optimize overall system efficiency, while satisfying required performance constraints.